Probiotics are live microorganisms which, according to the World Health Organization (WHO) when administered in adequate amounts, confer a health benefit on the host. Particularly, lactobacillus acidophilus bacteria promote intestinal health which is a major aspect of the host's general well-being. Probiotic products require some process steps that ensure their stability and viability. They can be negatively influenced by storage and during their passage through the gastrointestinal tract upon consumption.
Many probiotic products are freeze-dried to preserve them until they are used. This means they are first deep-frozen and afterwards dehydrated in a vacuum. The freeze-drying process is crucial for maintaining the stability and viability of probiotic bacteria as food additive, food supplement or nutraceutical.
Technically, freeze-drying, also known as lyophilization, lyophilization, or cryodesiccation, can be defined as cooling of liquid sample, resulting in the conversion of freeze-able solution into ice, crystallization of crystallisable solutes and the formation of an amorphous matrix comprising non-crystallizing solutes associated with unfrozen mixture, followed by evaporation (sublimation) of water from amorphous matrix. The evaporation (sublimation) of the frozen water in the material is usually carried out by reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase. The great advantage of freeze drying is to stabilize the materials for storage.
Furthermore, freeze-drying has the advantage of no risk of thawing to the encapsulated cells (Santivarangkna, C., Kulozik, U. and Foerst, P. (2007) Alternative drying processes for the industrial preservation of lactic acid starter cultures. Biotechnology Progress, 23(2), 302-315). Significant mortality of bacterial cells has been reported after freeze drying due to the loss of membrane integrity and denaturation of macromolecules. See Franks, F. (1995) “Protein destabilization at low temperatures”. Advances in Protein Chemistry, 46, 105-139; Thammavongs, et al (1996) “Physiological response of Enterococcus faecalis JH2-2 to cold shock: Growth at low temperatures and freezing/thawing challenge” Letter in Applied Microbiology, 23(6), 398-402; De Angelis, M. and Gobbetti, M. (2004) “Environmental stress responses in Lactobacillus: A review” Proteomics, 4(1), 106-122.
It has been shown that encapsulation of bacteria in cellulose sulphate is able to exert a protecting effect on bacteria during freeze-drying. One study conducted with alginate-chitosan as encapsulation material mentioned that the capsules became swollen when they are incubated in simulated intestinal fluid (Paulraj Kanmani, R. Satish Kumar, N. Yuvaraj, K. A. Paari, V. Pattukumar, and Venkatesan Arul (2011) Cryopreservation and Microencapsulation of a Probiotic in Alginate-chitosan Capsules Improves Survival in Simulated Gastrointestinal Conditions. Biotechnology and Bioprecess Engineering, (16) 1106-1114. Kanmani et al. also describe in this study that sodium alginate-chitosan coated microcapsules shrank by 10% when in contact with simulated gastric fluid.
Thus, the detrimental action of the freeze drying process on cells such that bacterial cells might be offset by microencapsulation with sodium cellulose sulphate since such an encapsulation material may result in an improved stability of capsules and higher viability of encapsulated bacteria. However, there is still a need to overcome the detrimental action of the freeze drying process on cells such as bacterial cells.